This invention relates to a method and receiver for dynamically compensating for interference (e.g., adjacent-channel interference) to a frequency division multiplex signal.
Commercial radio broadcasters currently transmit with analog frequency modulation in the frequency modulation (FM) band from 88 MHz to 108 MHz in the United States. The FM band provides high-fidelity music, stereo music and other programming to numerous listeners.
The transmission of an FM hybrid analog/digital signal is contemplated with a digital in-band on-channel (IBOC) modulation scheme to provide digitally modulated programming with an improved audio signal-to-noise ratio and an enhanced audio dynamic frequency response. A digital-in-band on-channel modulation scheme refers to transmission of the digital signal within the existing FM band in a manner compatible with the present FM channel spacing. The digital signal of IBOC modulation scheme complements the existing analog signal to allow complete backwards compatibility with existing analog FM receivers. Further, a single radio station could offer a hybrid broadcast signal with different programming on a digital signal component, than on an analog FM signal component, potentially doubling the amount of programming offered by each broadcast station.
In the U.S., the Federal Communications Commission (FCC) assigns channels with an adjacent channel separation of 400 KHz between carriers in the same geographic region to maintain appropriate levels of signal coverage and to reduce adjacent channel interference. However, in certain areas as little separation as 200 KHz is present so interference between adjacent analog FM channels can be problematic even without the addition of a digital signal component.
For U.S. channel spacings, the FM modulated signal usually varies from approximately 0 HZ to 100 KHz on each side of a carrier frequency. Certain IBOC modulation schemes transmit digital side-bands in at least part of the regions between approximately 130 kHz to 199 KHz away from the carrier frequency. At a transmitter for transmitting an FM hybrid analog/digital signal, the analog FM signal may be filtered out so as not to interfere with the digital side-bands. However, at the receiver, interference from adjacent FM analog or FM hybrid channels may corrupt or destroy the desired digital sidebands. If the receive bandwidth of the digital transmission is restricted at the receiver, the digital transmission becomes less resistant to fading and modulated information may be lost. Thus, a need exists for a receiver that can dynamically reduce or eliminate. adjacent channel interference, with a minor or coextensive impact on fading tolerance.
In accordance with the invention, a method and a receiver for reducing adjacent-channel interference to a digitally modulated receive signal evaluates error rates of the receive signal associated with different receive bandwidths of the receive signal. The receiver selects a preferential bandwidth among the different bandwidths based on a suitably low error rate associated with the preferential bandwidth. The selection of the different bandwidths is achieved by digital signal processing that weighs desired data bits representing a desired bandwidth differently than rejected data bits representing a rejected bandwidth.
In one aspect of the invention, an identifier identifies the receive data bits associated with a rejection bandwidth or a desired bandwidth. The desired bandwidth is readily realized in the digital frequency domain by selectively puncturing receive data bits with substantially neutral logical values to yield rejected data bits associated with a rejection bandwidth of the receive signal. The initial received logical value of the rejected bits are discounted or devalued to the neutral logical value to facilitate the reduction of interference upon decoding of the desired data bits. A neutral logical value represents neither a high logic level nor a low logic level.